Boilers are widely used to generate steam for numerous applications. In a water-tube boiler, combustion of stoker or pulverized coal and coke, or gas or oil fuels provide radiation to the boiler tubes. Further, heat transfer is accomplished by arranging the flow of hot gases over the tubes to provide convection-heat transfer. In a typical low-pressure boiler designed to generate 200,000 lb/hr of steam at 235 psig and 500.degree. F., about 99.degree. F. of superheat is required since the saturation temperature at this pressure is only 401.degree. F. In some systems designed to generate the required amount of superheating, radiant boiler tubes cover an entire wall and roof surface within the boiler forming a "waterwall." With such systems, the temperature of the refractory walls is kept down, thus decreasing maintenance requirements. Often the water tubes are partially embedded in the walls. Typically, in this type of boiler, water is fed by gravity from the upper drums to headers at the bottom end of the waterwall tubes on all four radiant walls. Water circulation is upward through these tubes and the steam is disengaged from water in the upper drums of the boiler. The steam then passes through a steam separator before being superheated.
In a low-pressure boiler, the convection tubes reduce the flue gas temperature sufficiently such that the convection tubes can be routed directly to the air preheater, eliminating the need for a feed-water preheater sometimes referred to as an "economizer." The convection tubes are typically bent tubes running from the upper drums to the lower drums of the boiler. Water circulation in these tubes is, in general, downward in the cooler bank of tubes and upward through the hotter bank of tubes.
A typical power-generating steam boiler has a capacity of about 450,000 lb/hr of 900 psig steam delivered at about 875.degree. F. Since the saturation temperature at 900 psig is 532.degree. F., considerable superheating is required to obtain the steam delivery temperature. Because of the need for considerable superheat duty, little boiler convection surface can be placed between the radiant boiler and the superheater. This is because high-temperature combustion gases must be used to obtain the required superheat temperature while maintaining a reasonable superheater tube surface area. Since the feed water must be brought to the saturation temperature before it is admitted to the boiler drum, considerable heat is absorbed in the economizer section.
The thermal efficiency of the boiler can be further increased by preheating the combustion air with the flue gases before they are sent to the stack. In steam generating boilers, large amounts of fuel are needed for the combustion process. This is because of the need for superheating in order to achieve the required outlet steam temperatures of both low-pressure and power-generating steam boilers.
As the requirements for electrical energy continue to increase, improved operating methods are necessary in order to maintain fuel consumption and exhaust emissions within acceptable levels. Improvements in fuel combustion within steam generating boilers is one means to increase the operational efficiency of the boiler. However, any change in the combustion process within an existing steam-generating boiler must not take place without consideration of the thermodynamic processes within the boiler. For example, different heat transfer patterns within the various areas of the boiler such as the radiation zone and the convection zone, can lead to different localized vaporization/superheating rates of the steam. Nonuniform vaporization can lead to damage to the water tubes within the boiler. Additionally, non-uniform localized vapor superheating can lead to lower heat transfer coefficients, which can cause pipe overheating. Accordingly, when making alterations to the combustion process within the boiler, it is desirable to maintain relatively unchanged the originally designed heat transfer patterns within the boiler.
One method for increasing the efficiency of the combustion process is to use oxygen-enriched air as an oxidant. Oxygen-enriched combustion has been employed in numerous industrial applications such as glass, steel, aluminum and cement manufacturing. The use of oxygen-enriched air has led to significant process improvements such as fuel savings, production increases and expanded use of waste materials as fuel. Additionally, oxygen enrichment has been used for combustion in the lower central zone of recovery boilers in the pulp and paper industries.
The use of oxygen-enriched air is also employed in operation of boilers using coal-water-mixture (CWM). The results of experimentation conducted with a 700 HP water-tube boiler using bituminous CWM suggest that the use of oxygen-enriched air increased carbon burnout, reduced uncontrolled fly ash emissions and reduced combustion air preheating requirements. Additionally, the boiler efficiency increased because of reduced flue gas heat losses.
Although the use of oxygen-enriched air and oxygen-containing gases has been shown to improve boiler operation, further improvements are necessary to fully realize the increased operational efficiency potentially obtainable in large steam-generating boilers. The need to maintain thermodynamic balance within the radiation zone and convection zone of a large steam-generating boiler is necessary if existing boilers are to be retrofitted for oxygen enhanced combustion. Accordingly, a need exists for a method of operating a steam-generating boiler that fully utilizes oxygen enhanced combustion while maintaining parameters such as the flue gas mass flow rate and steam properties such as temperature, pressure, flow rate and the like within boiler design limits.